I'm one [of the few million] who has been there. ("Has" agrees with "one".)
[I'm one of] the few million who have been there. ("Have" agrees with "the few million".)
I have sat for the last few minutes having an argument with myself (!) as to whether I would say:
I'm one of the people who has been there
I'm one of the people who have been there.
Honestly, I can't tell you which one I would say in a natural conversation! Don't you love English?!